def f(n):
    for i in range (2,int(n**0.5)+2):
        if n%i==0: return False
    return True

b=[]
x=int(input())
if len(str(x))!=4:
    print(0) 
else:
     x1=x//1000
     x2=x//100%10
     x3=x%100//10
     x4=x%10
     x5=x//100
     x6=x%1000//10
     x7=x%100
     x8=x//10
     x9=x%1000
     a=[x1,x2,x3,x4,x5,x6,x7,x8,x9,x]
     for m in a:
         if f(m):
             b.append(m)
     if len(b)>0:
        
         print(max(b))
     else:
           
         print (0)